Tutorial on Field Instrumentation

A variety of field instrumentation is in wide use today by environmental engineers and consultants. Questions often arise regarding the theory of operation of various types of instruments and what is the proper or "best" application at a particular field site.

This article will provide a brief theory of operation for four common types of field instruments (four-gas instruments, photoionization monitors, flame ionization monitors, and portable gas chromatographs) and some typical applications for each. For maximum safety and accuracy the user must ensure any instrument used at a field site is working properly and has been calibrated according the manufacturer's instructions. Along with a properly working instrument, a trained operator who understands the benefits and limitations of an instrument is the key to the successful application of field instrumentation.

Combustible Gas Instruments
One of the most common instruments at almost all field sites is a combustible gas instrument (CGI). A CGI is not specific to any one flammable gas, but is designed to detect many gases. These instruments can have several types of detectors.

One of the most common detectors in a CGI is the catalytic sensor. The catalytic sensor is exposed to the sample. There is also a compensating sensor that is used as a reference and is not exposed to the sample. When the instrument is turned on, the catalytic sensor and the compensating sensor are heated to a very high temperature. As samples containing flammable gases are pumped or diffused across the catalytic sensor, the temperature across the catalytic sensor increases. The increase in temperature causes an increase in resistance and a proportional decrease in the current through the catalytic sensor.

This decrease in current is compared to the current through the compensating sensor, and a microprocessor is used to convert the current to a meter reading. For some CGIs, solid-state sensors are used in place of a catalytic sensor. The readout for both types of sensors can be presented either as a percentage of the Lower Explosive Limit (% LEL) or the percent gas concentration.

Because some gases will burn more readily than others on the catalytic sensor, the CGI will not have the same sensitivity to different flammable gases. A combustible gas instrument will detect the presence of many flammable gases that may be found in any type of confined space, such as a manhole. Discarded drums and gas cylinders can be sources of combustible gases. Combustible gas also can be detected in or near containers that have been used to store waste. For worker protection, a combustible gas instrument is an essential tool at almost all field sites.

Today, a CGI is often combined with other sensors into what is commonly called a four-gas instrument. Other sensors in a four-gas instrument are an oxygen sensor and up to two toxic gas sensors. Toxic gas sensors are specific to one gas, such as carbon monoxide, carbon dioxide, hydrogen sulfide, sulfur dioxide, chlorine, chlorine dioxide, nitrogen dioxide, and ammonia.

The oxygen and toxic gas sensors are electrochemical sensors. An electrochemical sensor consists of a semi-permeable membrane that allows the target gas to diffuse into the electrolyte. The electrolyte will produce ions in proportion to the concentration of the gas diffusing through the membrane as a chemical reaction takes place between the target gas and the electrolyte. The ion count is converted to an instrument reading in parts per million (ppm) for toxic gases and percent for oxygen. Most toxic gas instruments measure over a range of low ppm up to as high as ten to hundreds of ppm.

The main advantage of electrochemical sensors is they are inexpensive and rugged enough for use in most sampling environments. However, toxic gas electrochemical sensors are subject to interference from same-sized molecules that have chemical characteristics similar to the toxic gas the sensor is designed to detect. Electrochemical sensors can be poisoned by other compounds that may be present in the sample. The sensor membrane can be clogged by particulates in the sample and water vapor. Because the electrolyte in an electrochemical sensor is consumed during the chemical reaction with the target gas, the sensors must be replaced every one to three years, based on the manufacturer's recommendation or whenever the sensor fails to calibrate properly.

Four-gas instruments are most commonly used in confined space applications where there may be a build-up of flammable gases or low levels of oxygen. Either condition is potentially life threatening, and the threat must be clearly understood. By using a four-gas instrument, site personnel can determine whether a respirator is required to enter a confined space or whether the area must be ventilated prior to entry because of the presence of flammable gas.

Photoionization Detectors
Photoionization based monitors are one of the most popular types of field instruments. Photoionization detectors (PIDs) were first developed in the 1950s for use on laboratory gas chromatographs. During the 1970s, several manufacturers developed hand-held photoionization monitors for detecting the total volatile organic compounds (VOCs).

In photoionization, an ultraviolet lamp containing an inert gas is energized. Once the lamp is energized, photons are released and pass through a flat crystal window on one end of the lamp. When the photons strike a molecule of a compound with the proper ionization potential, the compound is ionized, which releases an electron. The detector cell contains a repeller electrode and collector electrode. The ions move in the electric field between the repeller and collector electrodes, generating a current proportional to the concentration of the ionized molecules in the detector. This current is converted to a digital readout by the microprocessor in the instrument.

The most common lamp used in a PID is a 10.6 eV lamp, which will ionize compounds with ionization potentials from 8.0 eV up to 10.9 eV. Several hundred organic compounds have ionization potentials in this range, which makes the PID an essential instrument for detecting a wide range of volatile organic compounds at a field site. Photoionization monitors are commonly used for detecting the presence of volatile organics compounds and a limited list of inorganic compounds in air, soil, or water.

Because photoionization monitors are not specific and will detect any ionizable compound, they are the instrument of choice for detecting any volatile organic as part of a site survey. Advantages of photoionization monitors are wide detection limits, small size, simple maintenance, and low cost. Disadvantages are sensitivity to high levels of water vapor in the sample.

Flame Ionization Detectors
Flame ionization detectors (FIDs) are not as common as PIDs but are essential in certain applications. In a flame ionization detector, hydrogen stored in the instrument is combined with the oxygen in ambient air drawn into the instrument by a pump. When the hydrogen and oxygen are at the proper ratio, a glow plug ignites the mixture.

A thermocouple is used to monitor the flame temperature. When a sample is passed through the flame by the pump, any combustible organic compounds are ionized. As in the PID, the detector cell contains a repeller electrode and collector electrode. The ions move in the electric field between the repeller and collector electrodes, generating a current proportional to the concentration of the ionized molecules in the detector. This current is converted to a digital readout by the microprocessor in the instrument. After the sample is passed through the flame, it is vented from the detector.

A flame ionization detector is strictly an organic compound detector. Simple saturated hydrocarbons, such as methane and ethane, possess high combustion efficiencies; FIDs respond very well to these types of compounds. The presence of substituted functional groups (amino, hydroxyl, or halogens) on a simple hydrocarbon reduce the FID's combustion efficiency and the FID's sensitivity to the compound. The FID will detect organic compounds with ionization potentials from 8.0 eV to 15.4 eV. FIDs are insensitive to water vapor and are an excellent instrument to have available at a field site for working with high-moisture-content samples. The main disadvantage to FIDs is the need to refuel the instrument with hydrogen on a periodic basis.

An FID will readily detect methane, so it is widely used both at landfills and for leak detection near natural gas pipelines. FIDs also have a wide detection range, typically 0.5 ppm up to 50,000 ppm. Because of its wide detection range, FIDs often are used for Leak Detection and Repair (LDAR) requirements according to EPA Method 21.

Portable Gas Chromatographs
Gas chromatography (GC) is another technology that was invented in the 1950s. In chromatography, a sample is forced through a column by a carrier gas (typically hydrogen, helium, or nitrogen). The column is a very fine glass or metal tube packed or lined with materials that will attract and briefly hold certain compounds, then release them back into the sample stream.

Over the length of the column, this attraction and release will separate a mixed sample stream into groups of a single molecule. At the end of the column, many different detectors are used to quantify the concentration of a particular compound. Possible detectors include PID, FID, thermal conductivity detector (TCD), and electron capture detector (ECD).

A TCD is used for detection of both organic and inorganic compounds, often at high concentrations. It will detect the permanent gases such as oxygen, hydrogen, carbon dioxide, and carbon monoxide. An ECD is ideal for halogenated hydrocarbon detection.

The time that a compound reaches the detector is called the elution time. A compound will always have the same elution time if the instrument programming is constant. The elution time is compared to a pre-programmed library stored in the GC. Compound identification is based on elution time.

Several instrument manufacturers offer transportable or portable gas chromatographs for field use. The advantage of a field GC is the ability to run samples on site, which can eliminate sample degradation and eliminates or greatly reduces the time and expense of sending samples to a lab for analysis.

The primary disadvantage is that many GCs require a skilled operator, although some manufacturers have eliminated the need for a skilled operator by programming the GC at the factory so when the GC arrives at the field site, very little set-up is required. Another disadvantage is the requirement to supply carrier gas to the GC. A few manufactures make portable GCs that have a built-in supply of carrier gas, which does away with the need to bring carrier gas to the site. Finally, FID-equipped GCs require a source of hydrogen to fuel the flame. GCs with a PID or ECD detector eliminate the need for hydrogen.

The requirements of a field site will dictate which instruments are needed to protect workers' safety and health. By understanding the theory of various instruments' operation and applications, users can more easily choose instruments that meet regulatory or health/safety requirements and provide timely notification of the chemical hazards that may be present.

This article originally appeared in the November 2003 issue of Occupational Health & Safety.